Supernova (redirected from Supernove)

supernova, a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than iron are produced in supernovas. Supernovas also are the principal heat source for interstellar matter and may be a source of cosmic rays. Recent discoveries have confirmed an underlying connection between supernovas and gamma-ray bursts (GRBs). Both are associated with the deaths of massive stars and they often happen nearly simultaneously. There is no generally agreed upon model for how a massive star explodes. However, the association with gamma rays has renewed interest in the role played by stellar rotation and magnetic fields.

Distribution of Supernovas

At peak intensity, a supernova can shine as brightly as the entire galaxy in which it occurs. Novas are less spectacular and more common; they increase in brightness only by a few thousand times, and several occur in our galaxy every year. Supernovas can occur in that small percentage of stars having a mass greater than 8 to 10 times the mass of the sun and perhaps in certain binary stars.

More than five supernovas have been observed to have occurred in our galaxy in the last thousand years, including the "guest star" in Taurus described by Chinese astronomers in 1054; Tycho's star in Cassiopeia, observed by Tycho Brahe in 1572; and Kepler's supernova in 1604. In 1885 the first extragalactic supernova was discovered telescopically in the Andromeda Galaxy; some 700 others have been observed since. In 1987 Supernova 1987A appeared in the Large Magellanic Cloud. It was the first supernova visible to the unaided eye since 1604, and its eruption marked the first time that neutrinos were detected on earth from such an event (see neutrino astronomy).

Theoretical Models of Supernovas

Type I Supernovas

In the 1930s Fritz Zwicky, Walter Baade, and Rudolph Minkowski developed several models of supernova events. In a star about to become a Type I supernova, the star's hydrogen is exhausted, and the star's gravity pulling inward overcomes the forces of its thermonuclear fires pushing the material outward. As the core begins to contract, the remaining hydrogen ignites in a shell, swelling the star into a giant and beginning the process of helium burning. Eventually the star is left with a still contracting core of carbon and oxygen. If the star, now a white dwarf, has a nearby stellar companion, it will begin to pull matter from the companion. In many stars the excess matter is blown off periodically as a nova; if it is not, the star continues to get more and more massive until the matter in the core begins to contract again. When the star gets so massive that it passes Chandrasekhar's limit (1.44 times the sun's mass), it collapses very quickly and all of its matter explodes.

Type II Supernovas

Type II supernovas involve massive stars that burn their gases out within a few million years. If the star is massive enough, it will continue to undergo nucleosynthesis after the core has turned to helium and then to carbon. Heavier elements such as phosphorus, aluminum, and sulfur are created in shorter and shorter periods of time until silicon results. It takes less than a day for the silicon to fuse into iron; the iron core gets hotter and hotter and in less than a second the core collapses. Electrons are forced into the nuclei of their atoms, forming neutrons and neutrinos, and the star explodes, throwing as much as 90% of its material into space at speeds exceeding 18,630 mi (30,000 km) per sec. After the supernova explosion, there remains a small, hot neutron star, possibly visible as a pulsar, surrounded by an expanding cloud, such as that seen in the Crab Nebula.

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supernova

Any of a class of violently exploding stars whose luminosity after eruption suddenly increases many millions of times above its normal level. Like novas, supernovas undergo a tremendous, rapid brightening lasting a few weeks, followed by a slow dimming, and show blue-shifted emission lines on spectroscopy, which implies that hot gases are blown outward. Unlike a nova, a supernova explosion is a catastrophic event for a star, leading to its collapse into a neutron star or black hole. Amounts of its matter equal to the mass of several Suns may be blasted into space with such energy that the exploding star outshines its entire home galaxy. Only seven supernovas are known to have been recorded before the 17th century, the most famous in AD 1054; its remnants are visible today as the Crab Nebula. The closest and most studied supernova in modern times is SN 1987A, which appeared in 1987 in the Large Magellanic Cloud. Supernova explosions release not only tremendous amounts of radio energy and X-rays but also cosmic rays; in addition, they create and fling into interstellar space many of the heavier elements found in the universe, including those forming Earth's solar system.

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supernova

a star that explodes catastrophically owing to either instabilities following the exhaustion of its nuclear fuel or gravitational collapse following the accretion of matter from an orbiting companion star, becoming for a few days up to one hundred million times brighter than the sun. The expanding shell of debris (the supernova remnant) creates a nebula that radiates radio waves, X-rays, and light, for hundreds or thousands of years

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supernova [¦sü·pər′nō·və]

(astronomy)

A star that suddenly bursts into very great brilliance as a result of its blowing up; it is orders of magnitude brighter than a nova.

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Supernova 

a star that has undergone a catastrophic explosion followed by an enormous increase in brightness. At maximum brightness, the luminosity of a supernova is a billion times greater than the luminosity of such stars as the sun, sometimes exceeding the luminosity of the entire galaxy in which the supernova is located. The maximum brightness of a supernova occurs approximately two to three weeks after the explosion. The brightness subsequently decreases gradually, diminishing by a factor of 25 to 50 during the following 100 days. In a galaxy such as ours, there is an average of one or two supernovae per century. The last supernovae in our galaxy were observed by Tycho Brahe in 1572 and by J. Kepler in 1604. It is possible that in our galaxy during the last three centuries there have been several more supernovae that were not observed because their light was strongly absorbed by interstellar dust. In observing a large number of galaxies simultaneously, modern astronomers discover 15 to 20 extragalactic supernovae each year. The term “supernova” is applied to these objects by analogy with the term “nova”; it stresses the much greater power of these outbursts.

Supernovae are divided into two types according to the nature of the change in brightness over time and to the supernova spectrum. Type I supernovae are usually three to five times brighter than type II supernovae, and their brightness diminishes more slowly after reaching maximum. The spectra of type II supernovae have wide emission lines—their most characteristic feature; type I supernovae have very broad absorption lines. Type II supernovae also differ in having spectra with wide hydrogen lines, which are virtually absent in the spectra of type I supernovae.

The discovery of the products of supernova explosions in the Milky Way Galaxy was of great significance in the study of supernovae. These products consist of gas envelopes, called supernova remnants, which are expanding at great speed, and starlike objects, called pulsars. The latter are rapidly revolving neutron stars, characterized by radio emission, which pulsate with a period equal to their period of rotation. Supernova remnants are sources of synchrotron radiation, which occurs when high-energy electrons are braked in the magnetic fields of the gas envelopes. Some supernova remnants are also sources of thermal X-ray emission with temperatures of 10⁶°–10⁷°K. The Crab Nebula, which is located at the position where Chinese and Japanese chronicles record a bright supernova in 1054, may be considered the most impressive of all supernova remnants in our galaxy. In addition to the odd, filamentous nebula, which is expanding at a velocity of approximately 1,500 km/ sec, this remnant has a pulsar with an emission period of 0.033 sec in the radio-optical, X-ray, and gamma-ray bands. In some respects, the supernova of 1054 cannot be considered either type I or type II.

An analysis of available observation data on supernovae and remnants enables us to give a general outline of the evolution of a supernova (typical parameters are shown in Table 1). When the supernova explodes, a significant share of the mass of the star (in some cases, possibly the entire mass) is converted into an envelope, which expands at velocities up to 20,000 km/sec. The increase in brightness is related to a large degree to an increase in the radius of the radiating surface. At maximum brightness, the supernova has a colossal radius, exceeding that

Table 1. Characteristics of supernovae
Parameters	Type I supernova	Type II supernova
Mass of the ejected envelope (in units of solar mass)..........	0.1–0.5	approximately 1
Expansion velocity at maximum brightness (km/sec) .........	10,000–20,000	5,000–15,000
Temperature at maximum brightness (°K) ................	15,000–20,000	10,000–15,000
Total energy of emission (ergs).......................	1049–1050	3 × 1048-3 × 1049
Kinetic energy of the envelope (ergs) ...................	1050-1051	2 × 1050-2 × 1051

of the sun by 20,000–40,000 times. As the envelope expands, its density decreases. With continuing expansion in the interstellar medium, the envelope begins interacting with interstellar gas, leading to the formation of a shock wave. This causes the envelope to heat up and lose velocity. In tens of thousands of years, the supernova remnant engulfs a volume of space with a radius of more than 10 parsecs; the space is filled with hot plasma at a temperature of approximately 10⁶°K. On the boundary of this space is a layer of cooler and denser interstellar gas entrained during the envelope’s expansion; the mass of this gas reaches several hundred times that of the sun (a typical example of such a supernova remnant is the Veil Nebula in the constellation Cygnus). After hundreds of thousands of years, the envelope’s expansion velocity decreases to a magnitude of the order of 10 km/sec, and the envelope can no longer be identified against the background of chaotically moving clouds of interstellar gas.

As of the 1970’s, astronomical theory is unable to give a definite explanation of the mechanism of supernova outbursts. It appears, however, that the supernova explosion may result from the instability that arises in the later stages of the evolution of a star. The two most probable mechanisms of the outbursts are the thermonuclear explosion of a degenerated carbon nucleus and gravitational collapse, that is, a catastrophic collapse of stellar matter toward the center of the star when the star’s thermonuclear energy is completely exhausted. In the latter case, it is assumed that under certain conditions the intense liberation of gravitational energy causes the exterior layers of the star to fly apart.

One of the most interesting aspects of supernova physics is the role supernovae play in the thermonuclear fusion of chemical elements and the transformation of the chemical composition of the Milky Way Galaxy. At the moment of explosion, a significant share of a supernova’s mass in the form of hydrogen and helium is converted by thermonuclear reactions into elements with greater atomic weights. In the explosion, conditions arise for the fusion of even heavier elements, including those of the iron group. As a result, the matter released by supernovae into the interstellar medium is enriched with heavy elements. During the early history of our galaxy, quite a large number of supernovae exploded, which substantially changed the Galaxy’s initial chemical composition. Observations show that the oldest stars in our galaxy contain 100–1,000 times less matter composed of heavy elements than the sun and other stars that formed later.

The origin of cosmic rays in the Milky Way Galaxy is also significantly linked to supernovae. It is assumed that the acceleration of cosmic rays occurs in the electromagnetic fields of pulsars and, partially, in the shock waves of expanding supernova envelopes.

REFERENCES

Shklovskii, I. S. Sverkhnovye zvezdy. Moscow, 1966.
Pskovskii, Iu. P. Novye i sverkhnovye zvezdy. Moscow, 1974.
Mustel’, E. R. “Vspyshki sverkhnovykh i termoiadernye protsessy.” Priroda, 1974, no. 12.

E. R. MUSTEL’ and N. N. CHUGAI